This invention relates to motor-driven positioning systems using optical encoders, and in particular to such systems using a brushless direct-current (DC) motor.
There are many systems employing motors for moving various objects. One example is described in the above-related applications, whose contents are hereby incorporated by reference, and relates to a postage meter system using five separate motors in a closed loop system for setting the digit printwheels simultaneously during postage value setting. Another example is a high-speed mail handling machine, which uses a plurality of motors for moving envelopes from a stack through a series of stations which shingle the envelopes, seal them, weigh them, stamp them with a postage meter mechanism, and then discharge them with or without sorting according to their destination. See, for instance, commonly owned U.S. Pat. No. 4,930,764. A related, also commonly-owned U.S. Pat. No. 4,935,078, describes how the various stations are coordinated to speed-up the processing. This requires the presence of motors that can be caused to follow accurate velocity profiles including starting at one particular time, ramping up to maximum velocity at a later particular time, maintaining that velocity for a certain time, then decelerating under controlled conditions, and finally stopping at a particular time and position. Such machines commonly employ microprocessor-controlled, closed loop systems with brushless DC motors using optical encoders both for positioning and for commutation. Appliance, November 1988, pages 88-92, and Machine Design, Aug. 11, 1988, pages 109-113, whose contents are hereby incorporated by reference, provide a complete description of current brushless DC motors, rotor-position sensors, stator-commutation logic, and the power driver circuitry required to operate such motors.
Present high resolution (greater than 100 cpr) optical encoders for such motors have disks whose slots are fabricated using optical/etching methods. A "U" shaped sensor is typically used where light is transmitted from an emitter on one side of the disk, through a pattern of spaced disk slots, to a detector on the opposite side of the disk. Quadrature output for sensing the direction of rotation of the motor is obtained by mounting in the sensor two separate emitter/detectors that are separated from each other by a distance that is modulo .alpha./4 mechanical degrees along the disk encoder track circumference, that is n.alpha.+.alpha./4, where n can equal 0, 1, 2, 3 . . . , and .alpha. is the slot to slot spacing. As the disk rotates past the sensor, TTL level, square wave, output signals are generated in first and second channels, with the signals in the second channel being 90.degree. out-of-phase with those in the first channel.
The above disk fabrication process for a high resolution encoder disk is more expensive than metal stamping or plastic injection molding of a low resolution disk. Also, a U-shaped sensor structure positioned on opposite sides of the motor-driven disk precludes including the emitter/detector as part of an adjacent printed circuit board (PCB), as well as requiring separate connections for the encoder channels and power. These connection costs are a not insignificant part of the encoder cost. Moreover, in high resolution applications such as the described mail machines, the high resolution encoder disk is typically mounted after the motor driver PCB, making field service replacement of the PCB mounted electronics beneath the encoder disk impractical. Still further, in this arrangement, the motor shaft must traverse the PCB, which leaves less room for electrical components.